Voltage losses and recombination mechanisms in organic solar cells

Organic semiconductors offer significant advantages as photoactive materials for solar energy conversion, such as ease of synthesis, processing and tunability of properties. However, their efficiency and stability are significantly lower than competing technologies and this hinders their widespread commercialisation. Organic semiconductors are characterised by the relatively soft nature of the materials along with a strong coupling between electronic and vibrational modes. The aim of this work is to understand how the molecular nature of the materials limits the attainable power conversion efficiency of a device. Properly understanding how the properties of the molecules affect the device performance will serve as the basis to develop new chemical compounds for efficient and stable organic solar cells. In this thesis I have focused on understanding the origin of the open-circuit voltage (V_oc) losses in organic solar cells. V_oc Corresponds to the difference between the quasi-fermi levels at the contacts in an illuminated solar cell at zero current flow and is one of the characteristics of the device that controls power conversion efficiency (PCE). I used a combination of modelling and simulation to explain the experimentally measured voltage losses in series of organic photovoltaic (OPV) devices, aiming to understand which properties of the molecules impact the open circuit voltage of the devices. The main results of this thesis are presented in two chapters: In Chapter 4, I introduce a model for the voltage losses in OPV devices based on the radiative and non-radiative decay of the lowest energy charge transfer (CT) state and discuss its use. This CT state is formed at the interface between the donor and acceptor molecules in bulk-heterojunction solar cells. Using the model, I highlight the properties of the CT state that best explain trends in the voltage losses of OPV devices. These properties are related to the chemical structure of the molecules and their spatial arrangement. Therefore, these results serve as guidelines for developing molecules that would reduce the open circuit voltage losses. In chapter 5, I explore how to reliably characterise the recombination of the free charge carriers in OPV devices using optoelectronic techniques. Using a device simulation tool, I show that a commonly used characterisation technique (transient photovoltage (TPV)) is often unreliable. From the conclusions of the simulation I developed a modified characterisation technique (transient photo-charge (TPQ)) that overcomes the limitations of TPV. In the last section of chapter 5, I show how the combined use of the model in chapter 4 and the optoelectronic techniques can help further understand the photo-generation and recombination processes in OPV devices. Overall using a combination of molecular and device simulation approaches, I explored the correlation between the molecular properties of the organic semiconductors and the losses in the OPV devices. This work helps refine our understanding of the voltage losses in OPV devices and how to improve them.Open Acces